This invention relates to optical detector for broad spectral detection covering the wavelengths from ultra-violet (UV) to near infrared. More particularly, this invention is related to the multicolor detector, which can detect the light wavelengths ranges from as low as UV to the wavelengths to 2500 nm covering the most of the sensing and communication wavelengths. This invention is related to the multicolor sensor arrays for multicolor imaging.
Solid-state imaging devices with higher resolution are used in many commercial applications especially camera and also for other light imaging uses. Such imaging devices typically comprise of CCD (charge coupled device) photo detector arrays with associated switching elements, and address (scan) and read out (data) lines. This CCD technology is matured so much that now days a million of pixels and surrounding circuitry can be fabricated using the CMOS (complimentary metal oxide semiconductor) technology. As today's CCD technology is based on silicon (Si)-technology, the detectable spectral ranges are limited to the wavelengths below 1 μm where Si exhibits absorption. Besides, CCD based imaging technique has also other shortcomings such as high efficiency response combined with high quantum efficiency over broad spectral ranges, required for many applications. One of them could be the free space laser communication where shorter (in visible ranges) and near infrared wavelengths are expected to be used. Photodiode array having broad spectral detection capability is expected to provide those features not available in today's CCD technology. With well designed of the array, appreciable resolution can also be achieved in photodiode array technology.
Photodiodes especially of p-i-n type have been studied extensively over the last decade for its application in optical communication. These photodiodes are for near infrared detection, especially the wavelength vicinity to 1310 and 1550 nm. Now a day, the photodetector speed as high as 40 Gb/s, as described in the publication by Dutta et. al. in IEEE Journal of Lightwave Technology, vol. 20, pp. 2229–2238 (2002) is achieved. Photodetector having a quantum efficiency as close to 1, as described in the publication by Emsley et. al. in the IEEE J. Selective Topics in Quantum Electronics, vol. 8(4), pp. 948–955 (2002), is also available for optical communication. InGaAs material is usually used as absorption material, and the diode is fabricated on the InP wafer. On the other hand, Si based photodiodes are used for detection of visible radiation. For covering broad spectral ranges, conventionally two photodiodes fabricated from Si and InP technology, discretely integrated, are usually used. Although wafer bonding can be used to bond Si and InP to cover the wavelengths from visible to near infrared, the reliability of wafer bonding over wide range of temperature is still an unsolved issue and a high-speed operation is not feasible with a wafer bonding approach. It is highly desirable to have a monolithic photodiode array, which could offer high bandwidth (GHz and above) combined with high quantum efficiency over a broad spectral ranges (UV to 1700 nm/2500 nm). For using especially in imaging purpose where CCD based device is used, the photodiode array with the possibility to rapidly and randomly address any pixel is also very much essential.
It is our objective to develop a monolithic photodiode array for broad spectral ranges covering from UV to 1700 nm/2500 nm wavelength detection with having frequency response as high as 8 GHz and above bandwidth, and high quantum efficiency over 85% over the entire wavelength region.
Our innovative approach utilizes surface-illumination type (top or bottom-illumination type) photodiode structure having single or multiple absorption layer which can provide broad spectral response. The absorption layers will be designed to achieve the required quantum efficiency and also required speed. In array, to make possible to rapidly and randomly address any pixel independently, we will utilize the metal line to connect each pixel separately to the outside contacts pads for top-illumination type detector-array and etch-off substrate (thinning substrate) with flip-chip bonded type individual photodiode element for bottom-illumination type detector array. In top illumination detector array vase, as each metal line usually needs to connect outside pads to inside photodiode pixel, the pitch and element size is limited by the width of the metal line and array number. For example, the element size (i.e. photodiode pixel size) and pitch can be made to 5 μm and 10 μm, respectively, for the array size of 25×25 and metal line of 1 μm. More than 1000×1000 array could be possible for both top and bottom-illumination type detector structures.
According to the current invention, photodiode having broad spectral ranges, high quantum efficiency (>85%), and high frequency response over 10 GHz @ 3 dB, can be fabricated using the single wafer. According to this invention, in the case of photodiode array, each array can also be operated independently. The manufacturing thereof is also simpler as compared with the prior art. Some applications include multicolor imaging applications such as for astronomical observation, imaging etc. Other applications include communication and sensing.